iMac Headphone interferance

My headphone socket went on my work iMac shortly after the warranty run out which meant I couldn't listen to music. Imagine 8 hours of listening to work drivel...
I combated this by getting a USB Sound card from eBay for a couple of quid.

Intermittently I get noise interference which is deafening to the max and I have no idea where it comes from. It's only just started happening after about a year, I've just bought a new USB sound card, tried it in different ports and it's still doing it.
I might have to leave my job if I can't sort this.
Any ideas people?